1. Field of the Invention
This invention relates to an optical receiver such as may be used to terminate an optical fiber transmission line, or respond to an optical beam in free space.
2. Description of the Prior Art
Conventional high-speed optical receiver design is based on the well-known "high impedance" or "integrating" approach, an example of which is shown in FIG. 1. Here the light beam from an optical fiber, or received via free space, falls on a PIN diode 1 connected to the gate of an FET 2 having a drain resistor 6 and whose output, taken from the source, is coupled via an equalizing capacitor 3 to a broadband amplifier 4. The output of this amplifier 4 goes to the detection circuitry of the receiver. To achieve minimum noise, the load resistor 5 (R.sub.L) in series with the diode 1 is large in value, so the frequency response of the front end rolls off at -6 dB/octave through the frequency band of operation, and the signal at the input node (i.e. the gate of FET 2) takes a heavily integrated form. In such an arrangement a voltage swing of about 50 mV is allowable for an integrated waveform.
Such a receiver has to be operated with a digital optical line code of bounded digital sum variation (DSV), i.e. a code in which the cumulative disparity between the total number of received ones and zeroes is limited to a given small number, e.g. 4. Hence although the average phot-current may cause a high standing voltage drop across R.sub.L when the received optical power is large, the signal swing at the amplifier input node remains a small fraction of this drop and can thus readily be handled by the following amplifier before effecting equalization via a C-R differentiator. This gives good sensitivity, due to the large value of load resistor 5, and a reasonable dynamic range, but is only operable with a bounded DSV line code. However, at higher speeds, codecs for bounded DSV codes are relatively complex, so it is desirable if possible to operate systems with unbounded DSV codes. The high impedance receiver then suffers a severe "trade-off" between needing a large load resistor for sensitivity, but a low load resistor to limit the swing to a manageable level to achieve a workable dynamic range, so overall performance is poor.
An alternative conventional approach to the high-impedance receiver is the transimpedance configuration, in which the load resistor is in the feedback path of the receiver amplifier. This is normally so designed so as to achieve a flat overall frequency response across the required band of operation, and owing to practical component limitations this invariably results in the use of a lower value of load resistor than would be possible for an integrating receiver, and hence a significant sensitivity penalty. In addition, parasitics and component limitations make the transimpedance receiver, with its feedback around three active devices, increasingly difficult to realize where bandwidths of greater than a few hundreds of megahertz are required.